Widebeam antennas are used extensively in military and commercial consumer low-power applications. In general, they may consist of a dielectric waveguide opening with specially shaped conducting and dielectric boundary conditions. The radiating modes of the waveguide determine the far field radiation pattern of the antenna, which, for simple geometries, can be calculated via a Kirchoff diffraction integral. The theory of waveguide antennas is reviewed in Kraus, J., "Antennas" Second Edition, McGraw Hill, 1975.
One outstanding problem in the design of waveguide antennas has been the achievement of uniform hemispherical spatial coverage, while maintaining small size and low weight. More specifically, a circularly polarized, axially symmetric beam radiator is required in the microwave and millimeter wave frequency range. Some examples might be telemetry, tracking and command antennas used in connection with a satellite or a flying drone, antennas for aircraft microwave landing systems, SOS rescue, GPS (Global Positioning System) navigation, and compact efficient feeds for circular aperture antennas.
In the low frequency range, cross-dipoles, conical spirals and arrays of diffracting slots have been used to achieve widebeam radiation with some success. Such structures are not adaptable to the microwave and millimeter wave regimes because of structure complexity, tight fabrication tolerances and high losses.
Alternatively, at quasi-optical frequencies, approaches to the design of widebeam radiators have focused on divergent lenses and reflectors, which yield antennas too large and heavy for many of the applications mentioned. See, E. A. Lee and Y. M. Hwang, "An EHF Omnidirectional Lens Antenna", IEEE AP-S International Symposium 1989, p. 1610.
In the microwave and millimeter wave regimes, one approach to achieving hemispherical widebeam coverage is to taper the opening of the waveguide and simultaneously to control the cutoff frequency of the waveguide using a dielectric loading element. This approach usually yields narrow bandwidth and asymmetry in the radiation pattern.
Improved techniques proposed in conjunction with or in lieu of waveguide opening reduction include parasitic probes, U.S. Pat. No. 3,778,838, multiple cross dipoles and parasitic radiators suspended in front of the waveguide opening and a conical ground plane. See F. Boldissar and L. A. Alfredson, "A Ku-band Antenna for Spacecraft telemetry and Command", IEEE Antennas and Propagation Symposium, June 1984, p. 155 and A. Kumar, "Hemispherical Coverage Antenna for Spacecraft", Electronic Letters, 1988, p. 631. These approaches yield complicated antenna structures with rigid constraints on tolerance.
Finally, we are aware of an effort to achieve a broadbeam hemispherical uniform radiating structure in the X band using a specifically configured dielectric plug. See, E. G. A. Goodall, "Hemi-isotropic Radiators for the S- or X-band", Proc. IEE, 1959, p. 318 and E. G. A. Goodall, "Improvements In or Relating to Very Short Wave Aerials", British Patent No. 808,941, 1959. The resulting design is limited to linear polarization and exhibits an asymmetrical radiation pattern.
A fundamental challenge in all waveguide widebeam antenna designs is to achieve uniformity of coverage over a hemisphere via relatively uncomplicated radiating elements with a full polarization diversity.